1. Field of the Invention
The present invention relates to a discharge lamp lighting device and, more particularly, a discharge lamp lighting device wherein the fluorescent discharge lamp is lit using a self-excited half bridge inverter.
2. Description of the Related Art
The conventional device for lighting the fluorescent discharge lamp uses a self-excited half bridge inverter wherein a pair of main switching transistors are employed to form a single ended push-pull circuit. This inverter uses a feedback transformer wherein an output current applied from the inverter is detected by the transformer and this detected output is fed back to the base of each of the main switching transistors of the inverter from a secondary winding of the transformer so that the main switching transistors can be alternately switched on and off responsive to the output detected.
FIG. 1 shows an example of the conventional discharge lamp lighting device in which the above-described inverter is used.
In order to control the dimming of the discharge lamp or to stabilize the light output of the discharge lamp, switching times at which main switching transistors Q1a and Q1b are switched on and off are controlled so as to control the frequency of oscillation. This is achieved by changing time constants of the time constant circuits which comprise condensers C2a, C2b and resistors R2a, R2b by means of a variable impedance element or the like.
In the case of the inverter of the above-described type, a potential difference exists between a transistor drive section for one of the paired main switching transistors and another drive section for the other transistor. Namely, potentials (d) and (e) at bases of the main switching transistors Q1a and Q1b are different from each other in the circuit shown in FIG. 1. This prevents time constant control from being simultaneously carried out relative to both transistors Q1a and Q1b. Therefore, photo-couplers PCa, PCb or a transformer is used as an electrical insulating means to carry out the time constant control relative to both transistors. However, these components are so expensive as to make the device expensive.
In addition, it is difficult to control the output of the self-excited half bridge inverter over a wide range.